<!DOCTYPE html>
<html>
<head lang="en">
    <meta charset="UTF-8">
    <title></title>
    <script src="./js/jquery-1.7.1.js"></script>
</head>
<body>
    <button id="btn">click</button>
    <script>
        var obj = undefined;
        console.log(obj == null)
    </script>
</body>
</html>